Messier 46 (also catalogued as NGC 2437) is an open cluster in the constellation of Puppis. This is is very rich cluster, with its 150 stars of mag. between 10 and 13. The brightest of these stars are of spectral type A0, and each is about 100 times more luminous than the Sun. The age of M 46 is estimated to be about 300 million years. The stars of the cluster are scattered over about 27', which corresponds to a linear extension of 30 light years at the cluster's distance of 5,400 light years. A special feature in this cluster is the planetary nebula NGC 2438 (also catalogued PK 231+04.2) which appears within the borders of M46 (see bottom left image). However, this nebula is most probably not a member of the cluster. In this image you can spot another planetary nebula - PK 231 +4.1, which is smaller and dimmer than NGC 2438 (bottom right). You can see both planetaries together in the cropped image.